The present invention relates to light emitting diodes and in particular relates to packaged light emitting diodes that emit white light.
Light emitting diodes (LEDs) are a class of photonic semiconductor devices that convert an applied voltage into light by encouraging electron-hole recombination events in an appropriate semiconductor material. In turn, some or all of the energy released in the recombination event produces a photon.
Light emitting diodes share a number of the favorable characteristics of other semiconductor devices. These include generally robust physical characteristics, long lifetime, high reliability, and, depending upon the particular materials, low cost.
A number of terms are used herein that are common and well-understood in the industry. In such industry use, however, these terms are sometimes informally blended in their meaning. Accordingly, these terms will be used as precisely as possible herein, but in every case their meaning will be clear in context.
Accordingly, the term “diode” or “chip” typically refers to the structure that minimally includes two semiconductor portions of opposite conductivity types (p and n) along with some form of ohmic contacts to permit current to be applied across the resulting p-n junction.
The term “lamp” is used to designate a light emitting diode that is matched with an appropriate electrical contact and potentially a lens to form a discrete device that can be added to or included in electrical circuits or lighting fixtures or both.
As used herein, the term “package” typically refers to the placement of the semiconductor chip on an appropriate physical and electrical structure (sometimes as simple as a small piece of metal through which the electrical current is applied) along with a plastic lens (resin, epoxy, encapsulant) that provides some physical protection to the diode and can optically direct the light output. In the present context, the package includes a reflective structure, frequently formed of a polymer with in which the diode rests. Adding a lens and electrical contacts typically forms a lamp.
Appropriate references about the structure and operation of light emitting diodes and diode lamps include Sze, PHYSICS OF SEMICONDUCTOR DEVICES, 2d Edition (1981) and Schubert, LIGHT-EMITTING DIODES, Cambridge University Press (2003)
The color emitted by an LED is largely defined by the material from which it is formed. Diodes formed of gallium arsenide (GaAs) and gallium phosphide (GaP) tend to emit photons in the lower energy (red and yellow) portions of the visible spectrum. Materials such as silicon carbide (SiC) and the Group III nitrides have larger bandgaps and thus can generate photons with greater energy that appear in the green, blue and violet portions of the visible spectrum as well as in the ultraviolet portions of the electromagnetic spectrum.
In some applications, an LED is more useful when its output is moderated or converted to a different color. In particular, as the availability of blue-emitting LEDs has greatly increased, the use of yellow-emitting phosphors that down-convert the blue photons has likewise increased. Specifically, the combination of the blue light emitted by the diode and the yellow light emitted by the phosphor can create white light. In turn, the availability of white light from solid-state sources provides the capability to incorporate them in a number of applications, particularly including illumination and as lighting (frequently backlighting) for color displays. In such devices (e.g., flat computer screens, personal digital assistants, and cell phones), the blue LED and yellow phosphor produce white light which is then distributed in some fashion to illuminate the color pixels. Such color pixels are often formed by a combination of liquid crystals color filters and polarizers, and the entire unit including the backlighting is generally referred to as a liquid crystal display. (“LCD”).
In the present application, the term “white light” is used in a general sense. Those familiar with the generation of colors and of color perception by the human eye will recognize that particular blends of frequencies can be defined as “white” for precise purposes. Although some of the diodes described herein can produce such precise output, the term “white” is used somewhat more broadly herein and includes light that different individuals or detectors would perceive as having a slight tint toward, for example, yellow or blue.
As noted above with respect to displays, light emitting diodes are increasingly being used for illumination purposes. In this regards, “indication” refers to a light source that is viewed directly as a self-luminous object (e.g. an indicator light on a piece of electronic equipment) while “illumination” refers to a source used to view other objects in the light reflected by those objects (e.g., room lighting or desk lamps). See, National Lighting Product Information Program, http://www.lrc.rpi.edu/programs/NLPIP/glossary.asp (December 2006).
Illumination, however, tends to require higher quantities of light output than does indication. In this regard, the number of individual photons produced by a diode in any given amount of time depends upon the number of recombination events being generated in the diode, with the number of photons generally being less than the number of recombination events (i.e., not every event produces a photon). In turn, the number of recombination events depends upon the amount of current applied across the diode. Once again the number of recombination events will typically be less than the number of electrons injected across the junction. Thus, these electronic properties can reduce the external output of the diode.
Additionally, when photons are produced, they must also actually leave the diode and the lamp to be perceived by an observer. Although the majority of photons will leave the lamp without difficulty, a number of well-understood factors come into play that prevent the photons from leaving and that can thus reduce the external output of an LED lamp (i.e., its efficiency). These include internal reflection of a photon until it is re-absorbed rather than emitted. The difference in the index of refraction between the materials in the diode can also change the direction of an emitted photon towards an object that subsequently absorbs it. The same results can occur for yellow photons that are emitted by the phosphor in a phosphor-containing LED lamp. In an LED lamp such “objects” can include the substrate, parts of the packaging, the metal contact layers, and any other material or element that prevents the photon from escaping the lamp.
As noted above, white light LEDs are increasingly being used to light displays for electronic devices. In many of these applications, the LEDs are mounted perpendicular to the face of the relevant screen. In this orientation, and instead of being directed at the rear of the screen itself, the LEDs are directed towards the edges of a light guide—often a planar piece of plastic—so that when the light from the LEDs enters the edge of the light guide, the light guide both diffuses the light and redirects some of it perpendicularly towards the plane of the display screen.
Light emitting diodes packaged for this purpose are referred to as side view surface mount LEDs or “sidelookers.” The conventional package for a side-view (sidelooker) diode lamp is molded out of a white plastic resin in a shape that forms a cavity around a metal strip (which forms the lead frame for a diode mounted in the package). The cavity generally defines sidewall angles between the respective sides. In the most common packages, the sidewall angles are near-perpendicular to the surface (floor) upon which the chip is mounted.
Although display lighting represents a frequent use of sidelooker LEDs, sidelooker applications are not limited to displays or horizontal orientations. Similarly, although many displays are back-lit, the invention is useful with other lighting geometries or arrangements as well.
It is commonly assumed that white plastic material is a true Lambertian (diffuse) reflector; i.e. it scatters light equally in all 2π steradians. Accordingly, conventional packages tend to maximize the size of the package floor (the back inside surface of the cavity in the package) in order to make it easier to position the LED chip in the package (i.e., more room). To the extent that any such assumptions are inaccurate, however, the effective output of the LED lamp will suffer accordingly.
In this regard, a specular surface is one that is mirror-like, while a Lambertian surface is one that follows Lambert's cosine law and thus demonstrates the same luminance regardless of the viewing angle. It will thus be understood (as is understood by those of ordinary skill in this art) that any given surface is usually somewhere on a continuum from perfectly specular to perfectly Lambertian.
In short summary, a number of factors can reduce the external light output of an LED lamp. Accordingly, a need exists for continued improvement in increasing the external output of such LED lamps.